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Photoelectric cells

A photoelectric cells or photovoltaic cell or solar cell is a primary device that converts light into electricity direct current (DC) using the photoelectric effect, when electrons are emitted from matter (usually from semiconductor as a silicon) after the absorption of energy from electromagnetic radiation such as X-rays or visible light. Assemblies of cells are used to make photovoltaic panels, photovoltaic modules, or photovoltaic arrays.

  

Photovoltaic (PV) cells are semiconductor devices, usually made of silicon, which contain no liquids, corrosive chemicals or moving parts. They produce electricity as long as light shines on them, they require little maintenance, do not pollute and they operate silently, making photovoltaic energy the cleanest and safest method of power generation.
Photons in sunlight hit the photoelectric cell, then part of them are absorbed by semiconducting materials, such as silicon. Electrons (negatively charged) are knocked loose from their atoms, allowing them to flow through the material to produce electricity. Due to the special composition of solar cells, the electrons are allowed to move in a single direction only. The complementary positive charges that are also created (like bubbles) are called holes and flow in the direction opposite of the electrons in a silicon photoelectric cell. Set of photoelectric cells converts solar energy into a usable amount of direct current (DC) electricity.

 

A typical silicon PV cell is composed of a thin wafer consisting of an ultra-thin layer of phosphorus-doped (N-type) silicon on top of a thicker layer of boron-doped (P-type) silicon. An electrical field is created near the top surface of the cell where these two materials are in contact, called the P-N junction. When sunlight strikes the surface of a PV cell, this electrical field provides momentum and direction to light-stimulated electrons, resulting in a flow of current when the solar cell is connected to an electrical load.
Regardless of size, a typical silicon PV cell produces about 0.5 - 0.6 volt DC under open-circuit, no-load conditions. The current (and power) output of a PV cell depends on its efficiency and size (surface area), and is proportional the intensity of sunlight striking the surface of the cell. For example, under peak sunlight conditions, a typical commercial PV cell with a surface area of 160 cm2 will produce about 2 watts peak power. If the sunlight intensity were 40 percent of peak, this cell would produce about 0.8 watts.

Photoelectric Cells are classified into three generations each of which became important.
At present there is concurrent research into all three generations.


1-st generation of PV cells - majority is wafer-based crystalline silicon cells. Mono-crystalline silicon is produced from a pure-grade silicon melting. From it mono-crystalline rods are drawn are then cut into thin slices, called wafers. Their average conversion efficiency is 17%. Multi-crystalline silicon is of a lesser grade than mono-crystalline silicon. It is casted in blocks and sliced. Average efficiency is lower -14%. Mono-crystalline silicon is slightly higher priced and slightly more efficient than multi-crystalline. Ultra-pure Poly-crystalline silicon is used for producing of photoelectric cells too. Poly-crystalline silicon (or semi-crystalline silicon, poly-silicon, poly-Si) is a material consisting of multiple small silicon crystals. Regularly it is used in microelectronic industry or semiconductor industry. Poly-crystalline cells can be recognized by a visible grain, a “metal flake effect”. In 2006, over half of the world's supply of poly-silicon is being used for production of photoelectric cells and PV modules (panels). Shortage of purified silicon restricting production growth of crystalline silicon cells for worldwide PV industry. There are only twelve factories of solar grade poly-silicon in the world (in 2008).
The first generation PV cells technologies are most highly represented in commercial production, accounting for 89.6% of 2007 production. First Generation technologies involve high energy and labor inputs that prevent any significant progress in reducing production costs. Single junction silicon devices are approaching the theoretical limiting efficiency of 33%

2-nd generation of PV cells – are called silicon thin film PV technology. Thin-Film Photovoltaic Cell (TFPV), is a solar cell that is made by depositing one or more thin layers (thin film) of photovoltaic material on a substrate. The thickness range of such a layer is wide and varies from a few nanometers to tens of micrometers. Thin-Film Phopovoltaic cells are usually categorized according to the photovoltaic material used. The most successful second generation materials have been cadmium telluride (CdTe), copper indium gallium selenide (CIGS-CIS), dye-sensitized solar cell (DSC), amorphous silicon and micromorphous silicon. These materials are applied in a thin film to a supporting substrate such as glass or ceramics reducing material mass and therefore costs. These technologies offers PV conversion efficiencies 8-10%, but under laboratory conditions efficiency rate up to 20% have been reached already. Particularly CIGS-CIS, DSC and CdTe offers significantly cheaper production costs.
Thin-film semi-transparent modules can be used for the glazing of buildings. Due to their lower efficiency rates and cost the combination with buildings noise insulation and thermal protection semi-transparent thin-film PV modules (or shading photovoltaic elements) in ceilings and facades produces electricity and control the intensity of indoor daylight, create pleasant atmosphere. The expenditures for building integrated photovoltaic are comparable to those of stone facades

3-rd generation of PV cells - also called advanced thin-film photovoltaic cell, is a range of novel alternatives to "first generation" (silicon p-n junction or wafer solar cells) and "second generation" (low-cost, but low-efficiency thin-film) cells. It is an even more advanced version of the thin-film cell.
Several new solar cell or photovoltaic technologies have been proposed or discovered in recent years, due to extensive research and development with a focus on finding more efficient alternatives to traditional silicon solar cells. Research and development in this area generally aims to provide higher efficiency and lower cost per watt of electricity generated.
Current research of the 3-rd generation of PV cells is targeting conversion efficiencies of 30-60% while retaining low cost materials and manufacturing techniques. There are a few approaches to achieving these high efficiencies:
  • Multi-junction photovoltaic cell (multiple energy threshold devices).
  • Modifying spectrum or light rays or concentration systems.
  • Use of excess thermal generation (caused by UV light) to enhance voltages or carrier collection.
  • Use of infrared spectrum to produce electricity at night.
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